Network modification in potassium borate glasses: Structural studies with NMR and Raman spectroscopies

We have examined the structural changes in the boron-oxygen network that result from chemical modification of glassy boron oxide by potassium oxide. By means of boron-11 dynamic angle spinning (DAS) NMR and Raman spectroscopies, we have identified changes in the structure resulting from conversion of three-coordinate baron to four-coordinate species. We observe coordination changes in two different 3-fold coordinate boron sites: those in six-membered boroxol rings and those in nonring BO3 units. The DAS NMR data show that the fraction of boron in nonring three-coordinate sites decreases as modifier is added. Modification of the boroxol rings is also evident in the Raman data. Assignment of the NMR resonances is made with the help of comparison to spectra of related crystalline potassium borates. Using B-11 DAS NMR and O-17 magic angle spinning (MAS) NMR, we do not detect significant formation of nonbridging oxygen through 35% added modifier. The changes in the boroxol ring breathing mode in the Raman spectra are attributed to changes in the coordination of the boron that compose the boroxol ring, but coordination changes also occur in the nonring BO3 units. We propose a model that combines these NMR and Raman data, in which modification of both types of 3-fold coordinate sites is included. This model predicts a contribution to the formation of triborate units from nonring BO3 boron, as well as a slightly higher preference for modification of these sites over the boroxol ring boron.